There is a desire to decrease carbon dioxide (CO2) emissions from industrial facilities and power plants as a way of reducing global warming and protecting the environment. One solution, known as carbon sequestration, involves the capture and storage of CO2. Often the CO2 is simply buried. It would be valuable if instead of simply burying or storing the CO2, it could be converted into another product and put to a beneficial use.
Over the years, a number of electrochemical processes have been suggested for the conversion of CO2 into useful products. Some of these processes and their related catalysts are discussed in U.S. Pat. Nos. 3,959,094; 4,240,882; 4,349,464; 4,523,981; 4,545,872; 4,595,465; 4,608,132; 4,608,133; 4,609,440; 4,609,441; 4,609,451; 4,620,906; 4,668,349; 4,673,473; 4,711,708; 4,756,807; 4,818,353; 5,064,733; 5,284,563; 5,382,332; 5,457,079; 5,709,789; 5,928,806; 5,952,540; 6,024,855; 6,660,680; 6,664,207; 6,987,134; 7,157,404; 7,378,561; 7,479,570; U.S. Patent App. Pub. No. 2008/0223727; Hori, Y., “Electrochemical CO2 reduction on metal electrodes”, Modern Aspects of Electrochemistry 42 (2008), pages 89-189; Gattrell, M. et al. “A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper”, Journal of Electroanalytical Chemistry 594 (2006), pages 1-19; and DuBois, D., Encyclopedia of Electrochemistry, 7a, Springer (2006), pages 202-225.
Processes utilizing electrochemical cells for chemical conversions have been known for years. Generally, an electrochemical cell contains an anode, a cathode and an electrolyte. Catalysts can be placed on the anode, the cathode, and/or in the electrolyte to promote the desired chemical reactions. During operation, reactants or a solution containing reactants are fed into the cell. In an electrolytic cell, voltage is then applied between the anode and the cathode to promote the desired electrochemical reaction. Note that the convention for designating the cathode and anode is that the cathode is the electrode at which chemical reduction occurs. This is different for electrolytic cells (also known as electrolyzers, devices in which electrical energy is supplied in order to force a chemical oxidation-reduction reaction) than for galvanic cells (such as fuel cells and batteries) in which a spontaneous electrochemical reaction produces electricity in an external circuit during normal operation. In either case, a chemical species acquires electrons from the cathode, thus becoming more negative and reducing its formal oxidation number.
When an electrochemical cell is used as a CO2 conversion system, a reactant comprising CO2, carbonate or bicarbonate is fed into the cell. A voltage is applied to the cell, and the CO2 reacts to form new chemical compounds.
One of the issues at present is that to obtain high currents, one needs to run the electrochemical cells for conversion of CO2 either at low voltage efficiency or with continuous additions of co-reactants. For example, Schmidt et al., Electrochemical Reduction of CO2 (available at http://www.sccer-hae.ch/resources/Talks/08_Krause_Power_to_Value.pdf; last accessed on May 17, 2016) report that over 6 volts need to be applied to achieve a current of 600 mA/cm2. This corresponds to an electrical efficiency of 21%. Verma, S. et al. Phys. Chem. Chem. Phys., 2016. 18: p. 7075-7084 find that they can achieve 400 mA/cm2 at a cell potential of 3 V (42% electrical efficiency) by continuously supplying 3 M KOH to the cell. Unfortunately, if the KOH is recycled, the KOH will react with CO2 to form KHCO3. Verma et al. find that the current drops to 40 mA/cm2 in KHCO3.
An electrochemical cell for the conversion of CO2 that achieves (a) reasonable energy efficiencies (at least 40%), (b) reasonable currents (above 150 mA/cm2), and (c) reasonable selectivities (above 50%), without needing to continuously introduce other reactants, would represent a significant advance in the electrochemical field.